There are many applications in which it is desirable to determine, with accuracy, the location of an acoustic source. For example, in the fracturing of oil or gas-bearing shale by injecting water and chemicals under high pressure (hydraulic fracturing or “fracking”) acoustic energy is produced and it is advantageous to know the positions at which these acoustic events take place.
Conventional sensor systems include acoustic arrays of hydrophones or geophones, commonly termed ‘passive seismic’ or ‘micro seismic’ detectors, and are typically deployed either in linear form inserted into a borehole within a few hundred meters of the point of injection of fracturing fluids, or in the form of a two dimensional grid on or just below the surface adjacent to the injection point. A limitation of this prior art is that the number of sensors that may be deployed is limited at typically less than one hundred, and their spatial locations are either constant or may only be changed by laborious and time-consuming re-deployment of the sensor array.
Another important application is in security applications such as intruder detection. It is desirable to detect breaching of a perimeter fence, sabotage or illegal tapping of oil and gas pipelines. Each of these events will result in an acoustic signal being produced which, if detected, would allow an alarm to be raised. In this case, in detecting an acoustic event knowledge of position is also desirable. However, the strength of an acoustic source is usually unknown and so the strength of a signal as detected by an acoustic sensor is not a reliable indicator of the distance between the source and the sensor and therefore of position.
It is therefore an object of at least one embodiment of the present invention to provide a method of locating an acoustic source that obviates or mitigates one or more said limitations of the prior art.